1. Field of the Invention
The invention relates to an improved method of preparing citral from geraniol/nerol under selected Oppenauer oxidation conditions.
2. Brief Description of the Prior Art
Citral (3,7-dimethyl-2,6-octadienal) is a valuable and widely-used ingredient of lemon flavor and fragrance compositions. It is also useful as an intermediate in the preparation of the ionone family of aroma chemicals and in the production of vitamin A.
It is usually made by the oxidation of geraniol, nerol or mixtures of geraniol or nerol. One widely-practiced oxidation process is vapor-phase air-oxidation which is carried out at a high temperature in the presence of a catalytic metal, see for example British Pat. No. 1,381,587. The harsh conditions of this process lead to the formation of numerous by-products which contribute to lowering the yield and aroma quality of the prime product.
In contrast, the Oppenauer oxidation process, a well-known chemical technique for converting alcohols to ketones or aldehydes (see "Organic Reactions," Vol VI, Chapter 5, John Wiley and Sons, Inc.), has potential for giving citral of excellent aroma quality and in high chemical yield because it is carried out under mild conditions--at low temperatures and in the liquid phase. Oppenauer oxidation in general involves the transfer of two hydrogen atoms to a hydrogen acceptor compound, usually a ketone or aldehyde. Although the mechanism of the transfer is a matter of speculation, it is catalyzed by the presence of aluminum aryloxides or aluminum alkoxides. Although a wide variety of aluminum alkoxides and aryloxides may be added to the Oppenauer oxidation reaction mixture to initiate catalysis, it has been suggested by Djerassi (Organic Reactions, supra., page 209) and others that the true catalyst in the oxidation is an aluminum alkoxide or aryloxide generated in-situ from the alcohol reactant itself upon undergoing an exchange reaction with the additive aluminum alkoxide/aryloxide.
In any event, the Oppenauer oxidation reaction is complex. There are occurring simultaneously a number of reverse and side reactions. The role of the catalyst structure in regard to the main and side reactions is not fully understood. One side reaction which is particularly troublesome in the oxidation of an allylic alcohol to the corresponding aldehyde, particularly when employing an aliphatic aldehyde as the hydrogen acceptor, is the so-called "crossed-Tishchenko" reaction between the hydrogen acceptor and the alcohol starting material to obtain the corresponding ester. In the oxidation of geraniol/nerol to citral, the by-product is the mixture of geranyl/neryl esters which may be difficult to separate from the desired geranial/neral. Another troublesome side reaction is the "simple Tischenko" reaction involving an ester formed by the reaction of two moles of the aliphatic hydrogen acceptor. This side reaction reduces the concentration of hydrogen acceptor and thereby lowers the conversion of the alcohol to the desired aldehyde product.
U.S. Pat. No. 4,055,601 describes the use of furfural as the hydrogen acceptor for the conversion of 3-substituted and 3,3-disubstituted allyl alcohols, including geraniol, to their corresponding aldehydes. Although this process offers a considerable improvement over the methodology of the prior art because furfural does not undergo an aldol condensation with the aldehyde product, leading to a reduction in the amount of aluminum catalyst used to catalytic quantities, there are still many important disadvantages that remain when it is applied to making aroma-quality citral. For example, to make a purified form of citral economically, one needs to distill the crude reaction mixture, thereby removing the large excess of unreacted hydrogen acceptor and co-product alcohol. Furfuryl alcohol boils at a temperature of 170.degree. C. and furfural at 162.degree. C. These two compounds are therefore difficult to separate from each other by distillation. Also, since they are relatively high boiling compounds, their distillation will result in some loss of citral (boiling point 229.degree. C.) which is heat-sensitive. Further, the use of furfural as a hydrogen acceptor is not advantageous in that it oxidizes readily in air and decomposes in the presence of a base, which complicates storage and handling. Furfuryl alcohol decomposes in acid media.
Thus, the process of the U.S. Pat. No. 4,055,601 is only advantageous if the furfural is used under conditions such that it is all consumed and the crude citral converted to pseudo-ionone. There is then a considerable amount of geraniol/nerol which remains unconverted and must be recovered separately from the furfuryl alcohol for return to the start of process.
Tertiary aldehydes, like furfural and benzaldehyde, have no alpha-hydrogens. If they acted as Oppenauer acceptors, they should not give Aldol condensation by-products. However, they are highly sterically hindered and there are no literature reports of the use of such a tertiary aldehyde as an Oppenauer oxidant in the preparation of another ketone or aldehyde, in spite of the report of Adkins, et al. (J. Amer. Chem. Soc., Vol. 71, p. 3622) that the oxidation potential of trimethylacetaldehyde is only a little less than furfural.
By the method of the present invention, pure citral of excellent aroma and flavor quality may be prepared in improved yields, in the presence of a particular kind of hydrogen acceptor and under Oppenauer conditions, with reduced amounts of the undesired by-product Tishchenko esters and almost complete conversion of the feed geraniol/nerol. The tertiary aldehydes employed are much more stable in air, bases, and acids than either furfural or its corresponding alcohol. Unreacted tertiary aldehydes employed herein and their co-alcohols have relatively low boiling points, which are at least twenty degrees apart, thus leading to superior recovery of pure citral by-product alcohol, and of any unreacted tertiary aldehyde by simple distillation. The recovered tertiary alcohol may be separately oxidized back to the aldehyde and the aldehyde reused in future preparations.